Purity >95%. Polo-like kinases (Plks) are serine/threonine kinases with key roles in cell cycling. GSK461364 is a potent, reversible inhibitor of Plk1 (Ki = 2.2 nM). It is less effective against Plk2 and Plk3 (Ki s = 860 and 1,000 nM, respectively) and at least 1,000-fold selective for Plk1 over a panel of 48 other kinases. GSK461364 dose-dependently halts cell cycling in diverse proliferating cancer cell lines and, at higher doses, triggers apoptosis. It appears to be more effective against p53-deficient tumors and is able to cross the blood-brain barrier. GSK461364 is effective in vivo, inducing tumor growth inhibition or growth delay in xenograft models in mice.